1 - Download the RDS certificates (root plus region-specific intermediate ones) bundle:
wget -O config/rds-combined-ca-bundle.pem https://s3.amazonaws.com/rds-downloads/rds-combined-ca-bundle.pem
2 - Add config/rds-combined-ca-bundle.pem
to the repository and redeploy to Heroku.
3 - Update the DATABASE_URL
env var:
'postgres://user:password@host/db_name?sslmode=verify-full&sslrootcert=config/rds-combined-ca-bundle.pem'
do not forget to surround with quotes
References: